Weld line across structural rib
- Reduced effective stiffness
- Crack initiation under cyclic load
- Hard to fix without gate or geometry change
DFM
Designing for Weld Line Strength: Injection Molding Design Rules
Weld lines appear where melt fronts meet and need gate and geometry planning.
DFM
Weld lines form where flow fronts meet
Knit lines can be cosmetic defects or structural weak zones depending on location.
Gate location and hole features steer where flow fronts collide.
Weld lines through screw bosses or snap hooks need explicit acceptance or redesign.
Glass filled resins can show weaker knit performance at weld lines on loaded features.
DFM
Quote this rule with your CAD upload
NetProto DFM review flags these geometry patterns before steel is ordered.
- Call out draw direction and cosmetic faces on the model.
- Note resin family even when the grade is not final.
- Tighten tolerances only on mating features that need them.
Acceptance
Hidden knit versus knit across load path
Move gates or features before steel locks weld location.
Weld line on non structural hidden face
- Cosmetic texture may hide witness
- Document acceptance on drawing
- Easier to release T1 with customer sign off
Related rules
Design guide
Jump to sibling rules before you freeze CAD.
Design guide hub
All DFM topics on one route.
Design guide hubDFM article
Long form engineer guide.
DFM articleGate location
Steer weld placement.
Gate locationGate types
Gate style and shear.
Gate typesDefects hub
Flow mark and knit context.
Defects hubVisual reference
Molding context for this rule
Images from the Forge media library illustrate typical programs.
Injection molded part design review on CAD
Injection molded part design review on CAD
Plastic housing with ribs and bosses
Plastic housing with ribs and bosses
Engineer measuring molded component
Engineer measuring molded component
FAQ
Weld line questions
Can mold flow analysis move weld lines?
Simulation informs gate moves before steel is cut.
Are weld lines always weak?
Not always, but loaded features need explicit review.
Does venting remove weld lines?
No. Vents fix air traps, not knit line location.
Next step
Mark load paths and cosmetic faces
Upload CAD showing where knit lines are unacceptable.